(a) Field of the Invention
The present invention relates to an adaptive frequency control apparatus and a method thereof. More specifically, the present invention relates to an adaptive frequency control apparatus and a method thereof that are necessary for the integration of radio frequency (RF) devices of different digital communication systems and a digital signal processor.
(b) Description of the Related Art
In digital communication, especially digital mobile communication systems, RF frequency control is an important factor that determines system performance. RF frequency control between a base station and a terminal of the digital mobile communication system is needed for the following two reasons.
One reason is that an error of reference frequency sources between the base station and the terminal must be compensated. The error of reference frequency sources can be easily corrected without affecting a deterioration of performance, because it is almost static when ignoring temperature deviations or aging.
The other is that the Doppler frequency deviation and other channel environments are changing due to the movement of the terminal. The frequency deviation from the Doppler effect is non-static, because it changes according to the specific central frequency and the moving speed of the terminal.
The change in the channel environment caused by the movement of the terminal affects the frequency control performance more, so there is a need for taking measures to cope with it. The related technology is disclosed in Korean Patent Application No. 1998-51033 (applied on Nov. 26, 1998) under the title of “Loop Filter Coefficient Dynamic Allocation Method”. This conventional method includes determining a loop filter coefficient adequate to each situation and properly applying it to the change in the channel environment so as to prevent a deterioration of the frequency control performance possibly caused by the Doppler effect and a sudden change in the channel.
The RF frequency control is usually called “automatic frequency control (AFC)”, because it is automatically performed with a modem algorithm, and there are two AFC methods as follows.
One method involves a directly calculation of a frequency error at a digital receiver to directly control an analog VCO (Voltage Controlled Oscillator), and the other includes a correction of the frequency error with a digital NCO (Numerically Controlled Oscillator) at the digital receiver.
These two methods have good and bad points of their own. In the mobile communication system, there are some cases where the terminal must be synchronized in both frequency and time with the base station. For this purpose, a control of the analog VCO is necessarily performed.
Recently, many methods for processing the frequency error at a digital signal processor have been suggested so as to minimize the interface between the RF processor and the digital signal processor. For example, Korean Patent Application No. 2001-16612 (applied on Mar. 29, 2001) discloses an automatic frequency tracking apparatus and a method thereof. But, correcting the frequency error at the digital receiver is problematic in that the frequency control range is confined according to the symbol rate. With a general algorithm, the digital receiver can correct a frequency error of up to about 10% of the symbol rate. To correct a frequency error of above 10% of the symbol rate, the frequency control range must be confined through RF frequency sweeping.
The conventional methods proposed for the frequency control have limitations as follows.
First, the structure supporting different RF systems is not considered. When RF systems A and B are used to control an RF VCO, and an RF system C is used to correct the frequency error at a digital receiver, for example, there are some probable problems as follows.
(1) When the VCO control range is 1 V for RF system A and 0.5 V for RF system B so as to have a same RF output frequency error at the antenna output, the control resolution of the RF system A is half the control resolution of the RF system B with the same control loop and algorithm of the RF system B, thus resulting in a deterioration of performance.
(2) When the frequency error required in the system standard of RF system A is less than that in the system standard of RF system B, the RF system A needs a more precise frequency control than the RF system B.
(3) For RF system C, which is used to correct the frequency error at the digital end, frequency sweeping of RF is needed when the frequency correction range is smaller than the frequency error range.
Beside the aforementioned problems, all the control circuits related to AFC must have a structure that can be easily reconfigured in software, so as to match different RF systems to a digital communication system. But this is impossible with the AFC structure designed for a specific system in the conventional methods.
Second, the conventional methods do not consider a real-time debugging for an RF controller that is necessary for the matching of the RF processor and the digital signal processor. In addition, there is a need for a structure of monitoring AFC-related circuits in real time to reduce the development period and to acquire optimal AFC parameters. However, the conventional methods cannot provide such a structure, because they have AFC circuits as a black box type that shows an input-output system alone.